Everyday citizens are biased

Everyday citizens are not objective. Studies show that when reading people's faces and body language people overestimate their ability to tell whether someone is lying or not. Those trained in psychology and/or the law (preferably law with relevant psychological training) would recognize their own biases and check against them.

Having everyday citizens decide cases means that the everyday citizens' irrational biases will win the day. Any racial bias would also skew trials and indeed criminal defendants of color are more likely to be convicted and prosecutions are less likely to be successful when the alleged victim is a minority.

This creates a sustained sample.

In court, the jury is meant to provide an objective group of every day citizens to determine whether the evidence provided in a case can accuse the defendant. It requires no specializations to determine if evidence is substantial enough for a case. Any normal citizen can put two and two together.

Creating a jury made of only professionals would create a bias. Since the jury consists of people who work in the field, they have likely developed an opinion on any matters relating to it. In court, those opinions may hinder their judgement. That is not to say a normal jury couldn't have biased members. However these juries are random and provide a sample where no one opinion will be shared among the entirety of the jury.